In many data communication applications, serializer and de-serializer (SERDES) devices facilitate the transmission of parallel data between two points across a serial link. Data at one point is converted from parallel data to serial data and transmitted through a communications channel to the second point where it received and converted from serial data to parallel data.
At high data rates frequency-dependent signal loss from the communications channel (the signal path between the two end points of a serial link), as well as signal dispersion and distortion, can occur. Ideally, without noise, jitter, and other loss and dispersion effects, a data eye at the receiver will exhibit a relatively ideal shape. In practice, the shape of the data eye changes with noise, jitter, other loss and dispersion effects, and with temperature and voltage variations. As such, the communications channel, whether wired, optical, or wireless, acts as a filter and might be modeled in the frequency domain with a transfer function. Correction for frequency dependent losses of the communications channel, and other forms of signal degradation, often requires signal equalization of the signal at a receiver.
Equalization through use of one or more equalizers compensates for the signal degradation to improve communication quality. Equalization may also be employed at the transmit side to pre-condition the signal. Equalization, a form of filtering, generally requires some estimate of the transfer function of the channel to set its filter parameters. However, in many cases, the specific frequency-dependent signal degradation characteristics of a communications channel are unknown, and often vary with time. In such cases, an equalizer with adaptive setting of parameters providing sufficient adjustable range might be employed to mitigate the signal degradation of the signal transmitted through the communications channel. Equalization might be through a front end equalizer, a feedback equalizer, or some combination of both. The shape of the data eye also changes due to equalization applied to input signal of the receiver. In some systems, equalization applied by a transmitter's equalizer further alters the shape of the eye from the ideal.
If a simple, analog front-end equalizer (AFE) is employed, the data eye operating margin improves. However, better performance might be achieved through use of a Decision Feedback Equalizer (DFE) in combination with an AFE. Classical DFE equalization optimizes for intersymbol interference (ISI) and opens up the vertical and horizontal data eye opening. In SERDES communication channels, DFE filtering is employed to cancel post-cursor ISI in the equalized channel's pulse response by subtracting the output of the DFE from an input signal. DFE filters include a number of taps, the number of which determines how well the post-cursor ISI might be cancelled. The longer the filter length (i.e., the more filter taps), the more ISI terms might be cancelled, but at the expense of increasing DFE filter complexity and power consumption.